1. Field of the Invention
The present invention concerns a micropoint type cold cathode.
In particular, the invention applies to mass spectrometers in which the heated electrical filament emitting electrons is replaced by a micropoint type cold cathode. Cold cathodes of this type are electron emitting devices manufactured using semiconductor processes.
2. Description of the Prior Art
Unfortunately cold cathodes of the above type offer poor performance, only around 10% of the points of an array of micropoints emitting electrons. This is due to the non-homogeneous nature of the array of points which is caused, among other things, by manufacturing tolerances.
Because the points are non-homogeneous, the electric field at the end of a point varies greatly from one point to another. Electron emission as a function of the electric field at the end of the point obeys an exponential law. The resulting non-homogeneous emission is disadvantageous and the disadvantage is increased for operation at "high pressure", for example at a pressure equal to or greater than 10.sup.-4 mbar. If one point emits more electrons than its neighbors it is more sensitive to the phenomena of positive ion return and arcing which damage the points.
An aim of the present invention is therefore to propose a micropoint type cold cathode which improves the homogeneity of electron emission from the micropoints.